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3D breakthrough Fancy watching a movie on your mobile phone, where figures leap out from the screen in 3D, rather as Princess Leia did in the 1970s blockbuster Star Wars?

US researchers say they have moved that vision a pixel closer with the development of a three-dimensional image display that can be viewed without special glasses and is intended for cellphones, tablets and watches.

Unlike the holographic projection used in George Lucas' movie fantasy, their small prototype display is flat, backlit and uses a technology called diffractive optics to give 3D images that can be viewed from multiple angles.

"For example, if you were to display a 3D image of planet earth with the north pole facing out from the screen, by turning your head around the display, you would actually be able to have a view of any country on the globe," says David Fattal, who led the team at Hewlett-Packard Laboratories in Palo Alto, California.

Diffractive optics meet a challenge posed by the human anatomy, Fattal and his colleagues say in today's Nature journal.

Humans view the world stereoscopically, meaning our two eyes see two slightly different images because they are separated by about six centimetres.

Effect is limited

Two-dimensional screening provides a single flat image, which means the two eyes both see the same picture on the screen; 3D imaging, therefore, has to present a slightly different image to each eye.

Glasses-based systems work by having two lenses that each polarise the light in different directions, or by having lenses of red and green. In the first case, the display has two simultaneous images, each with different polarisation; in the second, the two images have red and green outlines.

Current glasses-free systems, including some mobiles, use thin lenses called lenticules or parallax barriers that send an image towards each eye.

But the 3D effect is limited and can only be perceived if the viewer is positioned in a narrow zone so that the correct eye gets the correct image.

The best option would be Princess Leia-style holography. But right now, this cannot be used for images displayed at a normal video rate, as the demands in pixel density are just too great.

The new "autostereoscopic multiview display" uses a backlight whose surface has been etched with tiny refractors.

Each of these microscopic deflectors send individual points of light in specific directions. These individual pixels, put together, comprise the different images sent to each eyeball.

The demonstration models can send light in 14 distinct viewing directions, providing the 3D effect in an angle of 90 degrees at a distance of up to a metre.

Tests have been carried out with images or footage, at 30 frames per second, of flowers, a turtle or a corporate logo.

Challenges ahead

The scientists say the design can be ramped up to produce up to 64 directions, further widening the viewing zone.

Using glass of high refractive index, the field of view could be "close to 180 degrees" they add.

"This current prototype is completely transparent and we think that even using a modulating device to achieve video rates, we can still retain most of the transparency," Fattal says.

In commentary also carried by Nature, University of Cambridge computer specialist Neil Dodgson says major challenges still lay ahead before the system will be commercially viable.

The new illumination system has much smaller pixels than mobile devices today, so more work has to be done to ensure that picture quality is not lost.

Another hurdle is to have the device manufactured "reliably, robustly and in quantity" which may take years, says Dodgson, who also pointed to the expense of providing content filmed in 3D in order to provide the multiple images.